Nanostructures (e.g. comprising nanomaterials) are generally defined as structures having at least one physical dimension on the order of 100 nm or less. Nanostructures are of increasing interest due to their unique properties as a result of their extremely small scale, for example electronic properties due to quantum confinement of electrons, optical properties based on transparency and luminescence and material properties due to the increased surface area and edge effects. Nanostructures can comprise nano-particles, quantum dots, carbon nanotubes, nano-wires, fullerenes (i.e. bucky balls), polycrystalline materials with ultrafine grain sizes, and three dimensional (3D) structures either naturally occurring or man made (e.g. synthesized). Examples of naturally occurring nanostructures include spider webs, natural fibers, insect wings etc. Manmade nanostructures can be synthesized directly, for example by gas phase condensation, microstructure refinement through severe plastic deformation induced by ball milling or torsion straining, crystallization from the amorphous state, electrodeposition and sol-gel processing techniques. Examples of some manmade nanostructures include polymers, self-aligned-monolayers (SAMs) and aerogels. Many of these nanostructures comprise materials (e.g. polymers and organic materials) that are not stable at elevated temperatures and/or cannot support mechanical loads and/or are not stable in various chemical environments as often required in applications such as micro-filtering and/or catalysis.
The present inventive methods overcome these limitations by utilizing atomic layer deposition (ALD) to deposit conformal layers of inorganic structural materials onto and within the physical structure of nanostructure templates. The templates themselves can be nanostructures comprising materials (e.g. polymers and organic materials) that are not stable at the elevated temperature of a particular application. By sequentially depositing structural materials layer by layer at incrementally higher temperatures, starting at a temperature equal to or below the maximum temperature at which the template is stable, a robust nanostructure that is stable at an elevated temperature (e.g. higher than the temperature at which the template itself is stable) can be formed from the original template. The resulting nanostructure can comprise a composite of the template within a “shell” of ALD deposited structural layers, or the template can be removed (e.g. dissolved, desorbed, decomposed etc.) to create a nanostructure formed of the deposited structural layers. The self-limiting surface chemistry of the ALD process provides atomic control of film thickness and additionally provides the ability to conformally coat shadowed, non line-of-sight, high aspect ratio features within nanoscale three-dimensional structures, that can be impractical by other deposition methods.